1409 The Canadian Mineralogist Vol. 38, pp. 1409-1418 (2000) CESIAN BAZZITE AND THORTVEITITE FROM CUASSO AL MONTE, VARESE, ITALY: A COMPARISON WITH THE MATERIAL FROM BAVENO, AND INFERRED ORIGIN CARLO MARIA GRAMACCIOLI Dipartimento di Scienze della Terra, Università degli Studi di Milano, Via Botticelli 23, I-20133 Milano, Italy VALERIA DIELLA Centro CNR per la Geodinamica alpina e quaternaria, Via Botticelli 23, I-20133 Milano, Italy FRANCESCO DEMARTIN Dipartimento di Chimica Strutturale e Stereochimica Inorganica, Università degli Studi di Milano, Via Venezian 21, I-20133 Milano, Italy PAOLO ORLANDI Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria, 53, I-56100 Pisa, Italy ITALO CAMPOSTRINI Dipartimento di Scienze della Terra, Università degli Studi di Milano, Via Botticelli 23, I-20133 Milano, Italy ABSTRACT The occurrence of thortveitite, Sc2Si2O7, and bazzite, Be3Sc2Si6O18, in the miarolitic cavities of the granophyre of Cuasso al Monte, in Varese, Italy, is reported for the first time, together with additional data for the corresponding species from the granite of Baveno. Bazzite from both these localities is Mg-poor and contains notable amounts of cesium (up to 2.3 wt% Cs2O), which is not encountered in the corresponding samples from Alpine fissures, and which has been overlooked in the type material (from Baveno). These discoveries confirm the similarity between the Baveno and Cuasso al Monte occurrences, and suggest the possi- bility that scandium minerals are more widespread in granitic rocks than is inferred at present. The composition and paragenesis of these Sc-rich species indicate the importance of complexes in enhancing the different geochemical behavior of scandium with respect to the REE, and that of equilibria with species such as feldspars, where aluminum cannot be replaced by transition elements. Keywords: scandium, thortveitite, bazzite, Baveno, Cuasso al Monte, Italy. SOMMAIRE Nous signalons pour la première fois la présence de thortvéitite, Sc2Si2O7, et de bazzite, Be3Sc2Si6O18, dans les miaroles d’un granophyre à Cuasso al Monte, dans la province de Varese, en Italie, et nous fournissons des observations nouvelles sur les mêmes espèces dans le granite de Baveno. A chaque endroit, la bazzite a une faible teneur en Mg et est enrichie en césium (jusqu’à 2.3% de Cs2O, en poids), ce qui n’avait pas été signalé dans la bazzite de la localité-type (Baveno). Ce ne sont pas les caractéristiques d’échantillons de ces espèces récupérés des fentes alpines. Ces nouvelles observations confirment la ressemblance de Baveno et de Cuasso al Monte, et fait penser que les minéraux de scandium pourraient bien être plus répandus dans les roches granitiques que l’on ne le soupçonne présentement. La composition et la paragenèse des espèces à Sc montre l’importance de complexes aqueux à distinguer le comportement géochimique du scandium par rapport aux terres rares, et d’équilibres ioniques avec des espèces telles que les feldspaths, dans lesquels l’aluminium ne peut pas être remplacé par des éléments de transition. Mots-clés: scandium, thortvéitite, bazzite, Baveno, Cuasso al Monte, Italie. § E-mail address: [email protected] 1409 38#6-déc.00-2203-08 1409 27/02/01, 13:25 1410 THE CANADIAN MINERALOGIST INTRODUCTION We thus decided to carry out a more detailed investiga- tion of these minerals from Cuasso al Monte. At the The discovery of bazzite, Be3Sc2Si6O18, in miarolitic same time, one cotype specimen of bazzite in good crys- cavities of the Baveno granite, in the Lago Maggiore tals and samples of thortveitite from Baveno had be- area of Italy, by Artini (1915) followed closely come available to us, thereby providing the possibility Eberhard’s (1908, 1910) discovery of thortveitite in the of obtaining improved crystallographic and composi- Iveland region in Norway as the first known mineral in tional data on these minerals from both localities. which scandium is present as an essential constituent. The original find at Baveno remained the only one there X-RAY DATA until about 1980, although several occurrences of bazzite have been discovered in Alpine fissures since Bazzite 1939 (see Hänni 1980 for references). At the same time, additional scandium-rich species have been observed at A fragment of a tiny blue, bazzite-like crystal from Baveno: for instance, thortveitite Sc2Si2O7 occurs in Cuasso al Monte was examined using a Nonius CAD–4 very unusual specimens as tiny greyish or deep blue single-crystal X-ray diffractometer. Although the poor crystals, in miarolitic cavities (Orlandi 1990). Other quality of the crystal prevented the possibility of refin- important discoveries are the new minerals cascandite ing the structure, the data were sufficient for deducing CaScSi3O8(OH) (Mellini & Merlino 1982, Mellini et al. the Laue symmetry 6/mmm and the unit-cell parameters; 1982), jervisite NaScSi2O6 (Mellini et al. 1982), and these results are reported in Table 1. On this basis, the 2+ scandiobabingtonite Ca2(Fe ,Mn)ScSi5O14(OH) identity of the mineral from Cuasso al Monte with (Orlandi et al. 1998). For a detailed history of all such bazzite is confirmed; final proof was provided by quan- discoveries, see Gramaccioli et al. (1998). titative electron-microprobe analysis (see below). The In recent years, the scandium-rich species and their large unit-cell dimensions constitute a peculiarity of paragenesis have received attention. Such an increase bazzite from this new locality; the high value of c may of interest is witnessed by a number of recent discover- relate to the presence of notable amounts of divalent ies of scandium minerals (besides the literature concern- ions, such as Fe2+, substituting for Sc3+, but the high ing Baveno, see also Bergstøl & Juve 1988, Juve & value of a cannot easily be explained. Bergstøl 1990, Armbruster et al. 1995, Liferovich et al. 1997, 1998, Bernhard et al. 1998, Raade & Erambert 1999); furthermore, the geochemical behavior of Sc, which is distinct with respect to the rare-earth elements (REE), has been shown not to be due only to the differ- ences in ionic radii. Recent observations have empha- sized the importance of complexes of Sc in the depositing solutions (Gramaccioli et al. 1999a, b, 2000). On considering the granophyre from Cuasso al Monte, Lombardy, Italy, and from the nearby locality of Carona in Canton Ticino, Switzerland, and its rela- tionships with the Baveno granite (Bakos et al. 1990, Boriani et al. 1992), scandium minerals were expected to occur here for some time. However, in 1992 an inter- esting specimen was noticed by Mr. R. Appiani in a quarry (Cava Laghetto) at Cuasso al Monte: it consists of a group of very tiny (maximum length 0.1 mm) grey- ish blue hexagonal prismatic crystals with flat termina- tions and similar to bazzite from Baveno, perched upon “orthoclase” (microcline) in a miarolitic cavity, associ- ated with quartz, albite, and a black biotite-like mineral. Furthermore, in 1994, another mineral collector, Mr. F. Anderbegani, submitted to our attention a certain number of specimens of another mineral he had found in the same quarry, showing very tiny groups of whitish aggregates of crystals. These aggregates are perched upon “orthoclase”, are associated with tourmaline, fluo- rite, quartz and albite, and are similar to greyish thortveitite from Baveno. A preliminary energy-dispersion analysis showed the presence of notable amounts of scandium in both cases. 1409 38#6-déc.00-2203-08 1410 27/02/01, 13:25 CESIAN BAZZITE AND THORTVEITITE, CUASSO AL MONTE, ITALY 1411 The cotype specimen of bazzite from Baveno exam- persion spectrometer; the results are reported in Tables ined is a part of the old Bazzi collection, which is now 5 and 6. The system was operated at an accelerating preserved in the mineralogical collection of Diparti- voltage of 15 kV, a sample current on brass of 10 nA, mento di Scienze della Terra, University of Milan. From and a counting time of 10 or 20 seconds on the peaks, this sample, a very tiny fragment of a crystal was se- and 5 or 10 seconds on the backgrounds. Kaersutite (for lected, and very good data were obtained using a CAD– Mg, Na,Ti, K), spessartine (for Fe and Mn), omphacite 4 single-crystal diffractometer, permitting accurate (for Si and Ca), pollucite (for Cs), and zircon (for Zr) refinement of the structure (Demartin et al. 2000). The unit-cell values (Table 1) are in line with recent values obtained for bazzite from other localities, but are nota- bly different with respect to the previous set determined by Peyronel (1956) on type (or cotype) material as well. Such a difference is almost certainly due to the poor accuracy of the older measurements, especially reflected in the anomalously small value of c. Thortveitite For the other scandium-bearing mineral from Cuasso al Monte and all thortveitite specimens from Baveno, no single crystals providing reliable X-ray data could be found. The X-ray powder-diffraction patterns of two different samples of this mineral from Cuasso al Monte, obtained using a Gandolfi camera, are reported in Table 2, together with those of a sample from Norway and of pure synthetic thortveitite for comparison. The corre- sponding patterns obtained from four different samples of thortveitite from Baveno are reported in Table 3, and the unit-cell parameters, obtained from a least-squares fit of these powder data, are reported in Table 4. They do not differ significantly, and agree satisfactorily with the corresponding data on “classic” occurrences in Nor- way and Madagascar (Bianchi et al. 1988), as well with those for a pure synthetic sample of Sc2Si2O7. The composition of thortveitite is variable; in some cases, the mineral is almost pure, and in others it con- tains notable quantities of yttrium, zirconium, and the heaviest REE. As was shown by several authors (Smolin & Shepelev 1970, Voloshin et al.